What was found
APPROX 65 MILES SOUTH OF DEPARTURE AT FL 230, AFT CARGO SMOKE MESSAGE ON THE EICAS, SMOKE MESSAGE ON THE ARMING SWITCH, A AURAL BELL, AND A FLASHING MASTER WARNING. FOLLOWING THE QRH, BLEW THE FIRST FIRE BOTTLE. SECONDS LATER THE LIGHTS EXTINGUISHED. CREW REQUESTED TO RETURN. ON FINAL APPROACH, THE SMOKE MESSAGE CAME BACK ON FOR ABOUT 10 SECONDS, AND THEN EXTINGUISHED. DECLARED AN EMERGENCY, LANDED UNEVENTFULLY. CARGO BAY WAS OPENED AND NOTHING UNUSUAL WAS FOUND AND THERE WAS NO BURNING SMELL DETECTED. ONE SMOKE DETECTOR (P/N 72-111211-000) WOULD TEST OK BUT THE "AFT CARGO SMOKE" LIGHT WOULD GO ON AND OFF IF THE SWITCH WAS HELD IN TEST POSITION. REPLACED THE DETECTOR (S/N A1787).